Multilayer printed circuit boards, having an inner core member with an electrical circuit pattern thereon made from copper foil, and additionally having layers of electrical circuit patterns made from copper foil and interleaved with sheets of prepreg, have recently been used in various electronics industries for providing electrical circuits, including electrical circuits to be electrically connected to integrated circuit chips, and for supporting these chips. These multilayer printed circuit boards are formed using at least one member of fiber glass-reinforced epoxy resin (FR4 material) as an inner core or inner substrate, having electrical circuit patterns formed from copper foils along each side of the inner core, and the core and electrical circuit patterns being sandwiched by alternating layers of prepreg material and electrical circuit patterns, e.g., made from copper foil. Outer layers of the multilayer printed circuit boards include electrical circuit patterns made from copper foil. The layers of prepreg material are layers of fiber glass or other fabric which have been saturated with polymer resin and the resin partially cured (the resin being in the B-stage). In the produced multilayer printed circuit board, the polymer resin has been cured to the C-stage to form a laminate.
These multilayer printed circuit boards can be fabricated by the following processing steps. Initially, at least one inner core, of the FR4 material, is provided and copper foil is adhered to opposing sides thereof. Thereafter, the copper foil is etched to form desired electrical circuit patterns. Alternating layers of prepreg material and electrical circuit patterns (made from copper foil layers) are provided along both of the opposed sides of the inner core or cores having the electrical circuit patterns thereon, to form a stack, with at least one prepreg layer provided between adjacent layers of electrical circuit patterns. Copper foil forms the outer layers of these multilayer printed circuit boards, and these outer layer copper foils are etched to form desired electrical circuit patterns and/or connections for electrical components such as integrated circuit chips. The stack is subjected to heat and pressure to cure resin material of the prepregs so as to form a laminate of the stack.
The number of layers of prepreg material, and the total number of layers of copper foil (including layers of copper foil on the inner core, with each copper foil layer having been etched to form a desired electrical circuit pattern), can be as desired, but generally four or more layers of copper foil are utilized. Electrical connection to the inner layers of electrical circuit patterns is achieved by drilling via-holes in the formed laminate to the inner layers and providing conductive material in the via-holes, so as to provide electrical connections (1) between electrical circuit patterns of different layers of the laminate and (2) between electrical circuit patterns of the inner layers of the laminate and outside circuits, including components such as integrated circuit devices provided on the multilayer printed circuit board.
The inner core of FR4 material maintains the thickness of the circuit board, to be, for example, 0.62 mils or 0.90 mils.
A problem with conventional multilayer printed circuit boards is that the FR4 material of the inner core is expensive.